Very finely chop the tuna fillet. Peel and finely chop the shallots. Rinse the chives and pat dry. Cut half of the chives into very thin rings and mix in a bowl with the tuna, shallots and lemon juice, season with salt and pepper. Using 2 teaspoons scoop mounds of the tartare onto plates and refrigerate.

Working over a bowl to catch the juice, with a paring knife, follow the curve of the oranges and remove the peel and white pith. Cut in between the membranes to release the fruit.

Rinse the lettuce and spin dry. In a bowl, whisk together the vinegar, mustard and oil, season with salt and pepper. Arrange the lettuce, orange segments and tuna tartare on a serving dish or plate. Pour the vinaigrette over. Cut the remaining chives in half and sprinkle over the salad.